Circuits for determining very accurately the position of a device inside biological tissue

ABSTRACT

A method of and apparatus for determining very accurately the position of a device inside biological tissue comprising a detecting instrument with a probe, which probe generates a small magnetic field which can be disturbed by a magnetically permeable metal in the device inside the tissue when a narrow end of the probe is positioned immediately adjacent to tissue containing the metal, whereby after the metal-carrying device is inserted in the tissue, the end of the probe is used to precisely locate the device by scanning the tissue until the magnetic field is disturbed which causes the instrument to emit a signal.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application is related to the applications of Ronald S. Newbower,and William McCormick, which were filed on even date herewith.

FIELD OF THE INVENTION

This invention relates to a method of and apparatus for determiningaccurately the position of a device inside biological tissue,particularly the position of an endotracheal tube in the trachea orwindpipe of a medical patient.

BACKGROUND OF THE INVENTION

It is necessary to insure that the breathing passageway of certainmedical patients, e.g., those in surgery or intensive care, is kept openat all times. This is accomplished in the prior art by means of anendotracheal tube which is inserted through the patient's mouth or noseand extends through the patient's throat and into the patient's windpipeor trachea. These prior art tubes are hollow and open at both ends, andthe end that extends outside the mouth or nose is anchored in place,usually with tape. Air can then pass through the tube into and out ofthe patient's lungs.

The principal drawback of the prior art tubes is that the distal end ofthe tube inside the patient must be inserted to and kept at a relativelyspecific position which is at about the midpoint of the trachea. This isbecause if the tube is inserted too far into the trachea, its distal endmay extend into the bronchial tree for one lung, and thus the other lungwill receive no air and may collapse. On the other hand, if the end ofthe tube is not inserted far enough, it may interfere with the vocalcords, or it may enter the esophagus, which opens near the bottom of thethroat, and air would not reach the lungs. In a normal adult the tracheais about 11 centimeters in length, and the distal end of the tube isgenerally positioned at the trachea's approximate midpoint, and it maybe anchored in place by expanding a balloon attached to the tube. Thispositioning, however, has much less margin for error in children orinfants, whose tracheas are much shorter in length. Furthermore, forboth adults and children, even if the tube is properly positionedinitially, the movements of the patient often cause the tube to move upor down, and therefore the location of the distal end of the tube mustbe continuously monitored.

The prior art uses several methods for monitoring tube position. First,the tube position can be determined by x-ray, but notwithstanding thepossible adverse effect of continued exposure to x-rays, the principaldrawback here is that by the time the x-ray is taken, developed andreturned, the tube may have moved again. Accordingly, two real-timemonitoring methods are in wide use. They involve listening to the chestto hear if both lungs are filling and visual observation of the depthmarkings on the exposed tube. Neither of these real-time methods,however, is very precise.

SUMMARY OF THE INVENTION

I have discovered a method and an apparatus for determining veryaccurately the position of a device inside biological tissue,particularly the position of an endotracheal tube in the trachea of apatient. The invention comprises a detecting instrument which generatesa magnetic field which when disturbed causes the instrument to generatea signal, the field being disturbed by a highly magnetically permeablemetal attached to the device inserted into the tissue.

In a preferred embodiment, the probe has a central coil and two endcoils, all identical. When the instrument is turned on, the central coilis activated thereby setting up a magnetic field, and the voltageoutputs from the two end coils are compared in the detecting instrument.If the end of the probe comes adjacent to the metal attached to thedevice in the tissue, there is an imbalance in the field, and theinstrument produces a signal. At that point, the position of the probeon the skin is recorded by using a slide or template which slips downthe probe and is held in place of the probe on the patient's skin, whichis then marked by a pen through a central hole in the slide or template.The position of the device can then be monitored by replacing the probeon the indicated spot.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Drawings

I now turn to a description of the preferred embodiments, after firstbriefly describing the drawings.

FIG. 1 is a side view in partial section of the invention along with theendotracheal tube in place in a patient,

FIG. 2 is an enlarged sectional view of the probe of this invention, and

FIG. 3 is an enlarged sectional view of a coil arrangement of theinvention, and

FIG. 4 is a schematic diagram of the circuit of this invention, and

FIG. 5 is a schematic diagram of another circuit of this invention.

Structure

Referring to FIG. 1, a locating apparatus is shown at 10. Locatingapparatus 10 generally comprises a probe 20 and a detector instrument30, attached together by a flexible electrical cable 40.

Probe 20 generally comprises three coils L1, L2 and L3 co-axially woundaround a core 22 which is disposed inside a cylindrical casing 24, whichis shaped to act as a pointer. The core 22 has three annular grooves 26,27, 28 in which the coils L3, L1 and L2, each containing the same numberof windings, are located. However, coils with different numbers ofwindings may also be used. The end grooves 26, 28 are equidistantlyspaced from the central groove 27. In the preferred embodiment, thedistance between grooves is about 1 cm, and the grooves are each about 2mm wide and 2 mm deep. The core itself has a diameter of about 9 mm.Separate pairs of leads (not shown) are connected to each coil, and theleads are carried to the detector instrument 30 by cable 40. The frontend 29 of the probe 20 is opposite the cable end, and a slide 21 ismovably disposed on the casing 24. Instead of the slide 21, the probe 20may have a marking element attached to its front end 29. The end of thecore 22 is set adjacent to the front end 29 of the casing 24, but itcould be set at some distance inside the casing away from the front end29. The casing is preferably made of a non-magnetic material such asplastic, while the core 22 is made of phenolic resin impregnated linen.

The detector instrument 30 generally comprises a box 32 which is of asize that can easily be held in one hand. The box 32 has a switch SW1 onits front end, a speaker opening 34 on its back end, and a series oflights 36 and a calibration control knob 38 on its top. Detectorinstrument 30 contains a circuit 50 for the probe 20, which circuit 50is shown in FIG. 4.

The instrument 30 is detachable from the cable 40 at connector 42. Thisallows the probe 20 to be sterilized when needed, which sterilizationprocess would otherwise damage the instrument 30 and its circuit 50. Inaddition, since the probe 20 is detachable, different sized probes fordifferent types of patients, e.g., adults, infants, can be used with thesame instrument 30.

Referring to FIG. 4, the circuit 50 has a voltage source B1 connectedbetween ground and the switch SW1, shown schematically. The voltagesource B1 in the preferred embodiment is a 12.6 volt mercury battery.

A multivibrator U1 is connected across the voltage source B1 and switchSW1. The duty cycle of the multivibrator U1 is set by resistors R1, R2in series with C1. Capacitor C2 stabilizes a reference point in themultivibrator U1. In the preferred embodiment, multivibrator U1 is CMOS,and preferably an Intersil ICM7555. R1 is 10K ohms. R2 is 330K ohms, andC1 and C2 are 1000 picofarads and 0.01 microfarads respectively.

The output from multivibrator U1 is connected through resistors R3 andR4 to the bases of transistors Q1 and Q2. Capacitor C3 is connected toground from between resistors R3 and R4. The transistors Q1 and Q2,which are NPN and PNP respectively, are arranged as an emitter-followercircuit. In the preferred embodiment, Q1 is a 2N2222A, and Q2 is a2N2907A. Resistors R3 and R4 are 330 ohms and 220 ohms respectively, andcapacitor C3 is 1000 picofarads.

The collector of transistor Q1 is connected through resistor R5, whichis 22 ohms, to the positive side of the voltage source B1 (when switchSW1 is closed) and to ground through capacitor C4, which is 47microfarads. Capacitor C5 and resistor R6 are connected to the positiveside of voltage source B1. Capacitor C5 is also 47 microfarads, andresistor R6 is 100 ohms.

The emitters of transistors Q1, Q2 are connected to central coil circuit52. Central coil circuit 52 comprises capacitor C7 in series withresistor R7 and central coil L1 of the probe 20. Capacitor C6 isconnected across the coil L1. Capacitors C6 and C7 are 0.1 microfaradand 6.8 microfarads respectively and resistor R7 is 150 ohms.

The emitters of transistors Q1 and Q2 are also connected to voltagedoubler circuit 54. Doubler circuit 54 is comprised of capacitors C8, C9and diodes D1 and D2. Diodes D1 and D2 are both 1N4148, and capacitorsC8 and C9 are both 47 microfarads. Also, a resistor R8 of 100 ohms andcapacitor C10 of 47 microfarads in series are connected across capacitorC9.

The coil L1 is, as previously explained, the central or transmittingcoil on the core 22 of the probe 20. Receiving coil L2 is the coil atthe front end 29 of the probe 20 and receiving coil L3 is at theopposite end. As shown in the bottom portion of FIG. 4, coil L2 isconnected through capacitor C12 and resistor R11 to the negative inputof differential amplifier U2. This side of coil L2 is also connected toground through terminating resistor R9.

The coil L3 is connected to the positive input of differential amplifierU2 through capacitor C13 and variable resistor R12. This input circuitfor coil L3 also has a terminating resistor R10, and variable resistorR12 is connected to ground through resistor R13. Resistor R14 isconnected from the wiper of the variable resistor R12 to ground, andcapacitor C15 is in parallel with resistor R14. The feedback loop foramplifier U2 comprises the parallel combination of resistor R15 andcapacitor C14.

The amplifier U2 is a Texas Instruments TL081. Terminating resistors R9,R10 are both 220 ohms. Capacitors C12 and C13 are both 0.1 microfarads.Resistor R11 is 1K ohms, and variable resistor R12 is 20K ohms.Resistors R13 and R14 are 220K ohms each, and capacitor C15 is 120picofarads. The feedback loop resistor R15 and capacitor C14 are 220Kohms and 1000 picofarads.

The output of amplifier U2 is fed through capacitor C16 of 0.1microfarad and R16 of 220 ohms to the negative input of amplifier U3,the positive input of which is tied to ground. Diode D3, a 1N4148, isconnected between the output of amplifier U3 and its negative input. Thefeedback loop for the amplifier U3 comprises the parallel combination ofresistor R17 and capacitor C17 connected to the negative input and theoutput of the amplifier U3 through diode D4, also a 1N4148. Theamplifier U3 is a Texas Instruments TL082, and resistor R17 andcapacitor C17 are 220K ohms and 0.047 microfarads.

The output from amplifier U3 is coupled to ground through capacitor C18and resistor R18, 6.8 microfarads and 10K ohms. This output is also fedthrough R19 of 100K ohms to the positive input of non-invertingamplifier U4, which is also a Texas Instruments TL082. This positiveinput is also connected through resistors R21 to the wiper of variableresistor R20. Variable resistor R20 is connected between negativevoltage V- and through resistor R33 to ground. Resistors R21, R20, R33,and R35 are 330K ohms, 20K ohms, 100K ohms, and 390K ohms, respectively.

The feedback loop for amplifier U4 which is connected between thenegative input and its output comprises the parallel combination ofcapacitor C20 and resistor R24, 1 microfarad and 100K ohms,respectively, and includes resistors R22, R23. The negative input isalso connected to ground through resistor R22 and potentiometer R23,being 1K ohms and 50K ohms, respectively.

The output from amplifier U4 is fed to display driver U5 which isconnected to light emitting diodes D5 through D14, which comprise thelights 36 of detector instrument 30. Driver U5 is a NationalSemiconductor LM3915. Resistor R25 of 470 ohms is connected to thedriver U5, and filter capacitor C11 of 47 microfarads is connectedbetween the positive voltage line to the driver U5 and ground.

The output from amplifier U4 also is fed through resistor R26,potentiometer R27 to audio circuit 60. Audio circuit 60 comprisesunijunction transistor Q3, a speaker, (shown schematically), capacitor21 and resistors R28 and R29. The transistor Q3 is 2N1671. Resistors R26and R27 ar 4.7K ohms and 20K ohms respectively. Resistors R28 and R29are 4.7K ohms and 220 ohms, respectively, and capacitor C21 is 0.22microfarads.

In the preferred embodiment, it is desirable to include a low batteryvoltage circuit 70. Circuit 70 comprises a pair of transistors Q4 andQ5, both TIS97. The base of Q4 is connected to ground through resistorR34 of 47K ohms and to the positive voltage through zener diode D16 andresistor R30. Resistor R30 is 820 ohms. The emitter of transistor Q4 isconnected to ground, and its collector is connected to the base oftransistor Q5 and resistor R31, the latter being 10K ohms and connectedto the positive voltage source. The emitter of transistor Q5 is alsoconnected to ground, and its collector is connected to the positivevoltage source through resistor R32, of 820 ohms and light emittingdiode D15.

Operation

The general operation of the invention is as follows. As shown in FIG.1, an endotracheal tube 100 is inserted into a patient's mouth, and isextended down a desired distance into the trachea 200 in accordance withclinical judgment. The exposed portion of the tube 101 is fixed inplace. It is desirable that the distal end 102 of the tube 100 bepositioned at the approximate midpoint of the trachea 200, as shown,between the vocal cords 202 and the carina 207 of the bronchial tubes206 to the lungs. The tube 100 has a band 104 of metal foil near itsend, which band 104 is covered by plastic (not shown). The metal ispreferably mu metal sheet (available from Arnold Engineering ofFullerton, California) or equivalent. The distance between the band 104and the distal end 102 is selected so that for the particular type ofpatient, i.e., adult, child or infant, the band 104 will be positionedabove the sternal manubrium notch (not shown) when the tube is in place.

In order to determine the positioning of the tube's distal end 102. Thedetector instrument 30 is turned on and this induces a current flowthrough the central coil L1 of the probe 20 thereby creating a magneticfield encompassing the receiving coils L2 and L3. As the coils L2 and L3are balanced in terms of windings and distance from the central coil,any currents induced therein (in L2 and L3) will be the same. (This isnot precisely correct due to the inherent minor manufacturingdifferences in coils intended to be identical).

The front end 29 of the probe 20 is then positioned perpendicular to thepatient's throat area. When the probe end 29 is very near the metalband, which unbalances or distorts the magnetic field, the flux densitythrough coil L2 will increase, and the currents through coils L2 and L3will be unequal. This unequal current consequently causes the speaker tosound, and one of the light emitting diodes D5-14 to light. Due to thespeaker sound, the operator can focus his attention on the patient andthe probe and does not have to look at the instrument to determine ifthe band has been located. The frequency of the audible signal, and thediode lit depends upon the amount of voltage difference (e.g., the lastdiode D14 will light and the sound will be at the highest frequency whenthe probe is directly over the metal band).

The position of the band is then marked by sliding the slide 21 alongthe probe until it contacts the skin. The probe is removed while theslide is held in place on the skin. A pen is used to mark on the skinthe location of the band through the probe opening (not shown) in theslide.

To monitor the position of the tube 100, the front end of the probe isplaced on the marked spot. And if the maximum signal is not obtained,the tube has moved. The skin area is then scanned by passing the probeend over the surrounding area until the maximum reading is againobtained, and if necessary the tube is re-positioned so that itcorresponds to the original or desired position. Alternately, the probemay be held at the desired position, and the tube moved until a maximumreading is obtained again. No reading (audible signal and lights) isobtained if the probe is more than just above the skin or on the skinmore than a centimeter laterally away from the metal band. This isbecause the front receiving coil L2 has a small cross-sectional area andis relatively close to the transmit coil L1 and a relatively small fieldis used. As the receive coil L2 is small, the presence of the smallmetal band in the field will induce a substantial change in flux densitythrough the coil L2. Also, as the coil L2 is circular, the resolution ofthe device is improved as a maximum signal is obtained when the band isadjacent to the center of the small coil L2.

The specific circuit operation is as follows. The switch SW1 is closedto turn on the instrument. Multivibrator U1 then produces an outputwhich is a squarewave having a duty cycle of slightly more than 50%. Thefrequency of this signal is approximately 2K Hz for the preferredembodiment.

The multivibrator U1 is CMOS so as to minimize the current it draws(about 100 microamps). Accordingly, its output current drive capabilityis not very high, and the emitter-followers, transistors Q1 and Q2,provide the necessary current gain (of about one hundred in thepreferred embodiment). The interim circuit comprised of R3, R4 and C3increases the rise and fall time of the transitional edges of thesquarewaves so as to reduce switching noise. Resistor R5 and capacitorC4 decouple the transistor Q1 from the positive supply

The output signal from the emitters of transistors Q1 and Q2 drives thecoil circuit 52. Capacitor C7 a.c. couples the transistors' output(still a squarewave) through resistor R7 and through the central coil L1thereby creating the magnetic field. Capacitor C6 of this circuit 52damps some of the ringing due to the length of the cable between theprobe and the detector instrument which houses the rest of the circuit50 (except for the coils L1, L2 and L3).

The two other coils L2 and L3 are connected to differential amplifier U2which amplifies the difference between voltages across the two coils L2and L3. The outputs from the coils L2, L3 are unequal even in ano-metal-detected case due to the manufacturing tolerances of the coilsof the preferred embodiment. As R15/R11 determines the gain, the gain ofthe preferred embodiment is 220. In the amplifier circuit, capacitor C14defines the upper frequency of the bandpass of the amplifier U2.

As it is difficult to make coils L2 and L3 identical (as they are not,there will always be a small voltage difference between them) variableresistor R12 is used to correct out this error voltage so that theoutput voltage of amplifier U2 is nulled to a minimum output signal in ano-metal-detected condition. This can also be accomplished by mountingthe coils so as to permit adjusting of the field relative to the receivecoils. As shown in FIG. 3, for example, a transmit coil 80, receivecoils 82, 84 are stationary, and mounted on hollow cylinder 86. Cylinder86 is internally screw-threaded, and a metal slug 88 is movably mountedtherein. The adjustment is made by inserting a screw driver through anopening 89 in the front end of the cylinder and changing the axialposition of the slug 88 with respect to the coils 80, 82, 84.

The output voltage of U2, which is the amplified difference between thecoil voltages, is fed to the amplifier U3. Resistors R16 and R17determine the gain of this stage, which functions as a rectifiernetwork. The diodes D3 and D4 determine that the amplified outputvoltage from this stage is always positive. The output signal from thisstage is integrated by capacitor C18, which has an additional dischargepath through resistor R18 to reset more quickly the display lights whenthere is a reduced voltage output from U3.

The output of amplifier U3 which is a positive rectified d.c. signalafter capacitor C18, is then fed to non-inverting amplifier U4. Anegative d.c. voltage from resistor R20, which is filtered by capacitorC19, is summed with the amplifier U3 output before amplifier U4 giving anet voltage close to zero volts for a no-metal-detected state. Thissumming serves to eliminate any voltage due to the natural imbalance ofthe receive coils. Resistors R22, R23 and R24 determine gain.

The output from amplifier U4 is fed to the display driver U5. Dependingupon the level of that output, one of the L.E.D.s. D5-D14 willilluminate. These are set to light sequentially as the output from U4increases from a minimum to a maximum.

At the same time, the speaker circuit is activated, assuming a voltageoutput from amplifier U4. This circuit is an R-C unijunction-typeoscillator which, as the output from U4 varies, so does the chargingcurrent to capacitor C12 and the frequency of the tone of the speakerwill rise with increasing voltage signifying increasing proximity to themetal band.

The low voltage battery circuit works as follows. As long as the batteryvoltage is above the sum of the voltage drop across resistor R30 plusthe zener diode voltage plus Vbe of Q4, the transistor Q4 stays on andQ5 is off. If the battery voltage drops, Q4 turns off and Q5 turns on,lighting L.E.D. D15.

It should also be noted that V+ is supplied to the amplfiers in thepreferred embodiment by decoupling it from the positive supply byresistor R6 and capacitor C5. Likewise, resistor R8 and capacitor C10decouple the V- from conventional voltage doubler circuit 54.

OTHER EMBODIMENTS

Referring to FIG. 5, another circuit for the instrument 30 is shown at150.

An oscillator U6, having resistors R36, R37 and capacitor C22 as aninput stage, produces an alternating output current which is fed to atransmit coil circuit 152. Oscillator U6 is an NE555, and resistors R36,R37 are 10K ohms and 330K ohms respectively. Capacitor C22 is 0.0018microfarads.

Transmit coil circuit 152 comprises capacitor C23 in series withresistor R38 and transmitting coil L1'. Oscillator U6, which draws about10-12 milliamps, provides an output signal with sufficient outputcurrent capability to drive transmitting coil L1' which produces amagnetic field as in the preferred embodiment.

Also as in the preferred embodiment, two receiving coils L2' and L3' arewithin the magnetic field when the circuit 152 is activated. Receivingcoils L2' and L3', however, are connected in the polarity shown so thatthe net voltage output from the combination will be the difference inthe voltage dropped across each coil. This net voltage output is sentthrough resistor R39 and capacitor C24 to the positive input of anon-inverting amplifier U7. Capacitor C25 and resistor R40 areseparately connected between this input line and ground. The amplifierU7 is an LM747. Resistors R39 and R40 are 10K ohms and 100K ohmsrespectively. Capacitors C24 and C25 are 0.0068 microfarads and 0.015microfarads.

The feedback loop for amplifier U7 has resistors R41 and R44, both being100K ohms. Resistor R46 of 100 ohms is connected between the loop andground. Offset resistors R42, R43 are selected so that the output fromamplifier U7 will be zero if the voltage across the coils L2' and L3'are equal. The sum of the resistors R42 and R43 is about 100K ohms forthis embodiment.

The a.c. output voltage (if any) from the amplifier U7 is fed throughcapacitor C26 to the positive input for amplifier U8. As in thepreferred embodiment, the magnitude of this signal depends upon theamount of difference in the voltages across the receiving coils, andthat is dependent upon any increase in flux density through coil L2' dueto the proximity of the metal band.

Amplifier U8 and its circuitry act as a precision rectifier. Diode D17is connected between the output and the negative input of amplifier U8.Resistors R48 and R50 in series are connected between the negative inputof amplifier U8 and through diode D18 to the amplifier output. DiodesD17, D18 are 1N4009. Resistors R48, R50 are 100K ohms and 10K ohmsrespectively. Also, resistors R47 and R49 are connected to ground asshown. These resistors are 100K ohms and 330 ohms respectively.

The d.c. voltage output (if any) from the amplifier circuit is fedthrough resistor R51 of 100 ohms to the display circuit 154. Displaycircuit 154 comprises four L.E.D. circuits, one each for L.E.D.s D19,D20, D21 and D22. Referring to the circuit for LED D19, the d.c. voltagefrom the amplifier U8 is fed through resistor R52 to the base oftransistor Q6, thereby turning it on. A current flows through thetransistor and D19 and resistor R56 causing L.E.D. D19 to light. D20lights in the same manner, except that due to the presence of diode D23in its L.E.D. circuit, the voltage on the base of transistor Q7 must besomewhat higher to create a current flow through diode D20. Diode 21requires still more transistor base voltage for Q8 as its circuit hastwo diodes in it, diodes D24 and D23. Similarly, still more voltage isrequired for the three diode circuit of L.E.D. D22. The net effect isthat the L.E.D.s light in sequence, but unlike the preferred embodiment,all will be on for a maximum voltage condition (i.e., close proximity ofthe coil L2' to the metal band). Transistors Q6- 9 are all 2N5734.Resistor R52 is 1.8K ohms. Resistors R53 and 54 are both 1K ohms, andresistor R55 is 680 ohms. The diodes D23, 24, 25 are all 1N4009.Resistors R56, 57 are 470 ohms. Resistor R58 is 330 ohms and resistorR59 is 220 ohms. Filter capacitor C27 is 47 microfarads.

A circuit 156 comprised of resistor R60 in series with L.E.D. D26 andzener diode 027 is connected between the +V and ground. While the +Vlevel is sufficient, L.E.D. D26 will light. Resistor R60 is 220 ohms andthe zener diode is a 4.7 volt diode.

Battery circuit 158 provides the +V voltage. Battery circuit 158 has a 9volt battery B2, a switch SW2 and a filter capacitor C28 of 68microfarads.

The -V voltage is provided by doubler circuit 160, which is similar tothe doubler circuit 54 of the preferred embodiment. Here, capacitor C29is 22 microfarads and capacitor C30 is 47 microfarads. The diodes D28,D29 are 1N192.

Other uses and variations are possible. For example, in addition to usewith endotracheal tubes, the method and apparatus of this invention canalso be used with other medical devices, such as stomach tubes, trachealtubes, venus catheters, arterial catheters, surgical sponges, and othertypes of catheters and devices, the position of which in tissue isimportant. Further, other fields e.g., electrical fields with adielectric instead of a metal band on the tube, can be used in place ofthe magnetic field and the metal band.

Other variations will occur to those skilled in the art.

What I claim is:
 1. A circuit for determining very accurately theposition of a device in biological tissue, comprising:a detectorcircuit,said detector circuit having a field coil which, when activated,generates a field, said detector circuit also having a first receivingcoil and a second receiving coil which are both disposed in the fieldgenerated by said field coil, whereby said first receiving coil producesa first output signal and said second receiving coil produces a secondoutput signal when the field is generated, a variable resistor having awiper, a differential amplifier having first and second inputs and anoutput, a first side of said first receiving coil and a first side ofsaid second receiving coil being connected to a common reference, asecond side of said first receiving coil being connected to said firstinput of said amplifier, a second side of said second receiving coilbeing connected to said variable resistor, said wiper of said resistorbeing connected to said second input of said amplifier so that themagnitude of said second output signal may be varied so as to make itequal to the magnitude of said first output signal when no device isdetected in the field, whereby said amplifier provides an output signalwhen there is a difference between said first and said second outputsignals from said first and said second receiving coils, and analarm,said alarm being connected to receive the signal from saidamplifier output to produce an alarm signal when there is an outputsignal, as would be caused by the presence of the device inside thefield.
 2. The circuit of claim 1 wherein the field is magnetic and thedevice carries metal which distorts the field.
 3. The circuit of claim 1wherein said circuit is powered by a battery, and said circuit furtherincludes a low-battery voltage indicator circuit.
 4. The circuit ofclaim 1 further comprising a rectifier network means connected betweensaid amplifier and said alarm for providing a rectified amplifier outputsignal to said alarm.
 5. The circuit of claim 4 wherein a d.c. offsetvoltage is added to said rectified amplifier output signal at a summingjunction in circuit before said alarm.
 6. The circuit of claim 5 furtherincluding a variable resistor means for adjusting said d.c. offsetvoltage.
 7. The circuit of claim 5 further including a second amplifierconnected between said summing junction and said alarm for amplifyingany voltage appearing at said summing junction.
 8. The circuit of claim7 wherein said alarm is an audio speaker circuit adapted to produce anaudio output signal, the frequency of which increases with the magnitudeof the amplified output from said second amplifier.